As is known, heavy-duty tyre-changing machines generally comprise a sturdy support frame, on which two slides are mounted, provided with a reciprocal nearing and a distancing movement in a horizontal direction.
A wheel-bearing chuck is mounted on a first slide, which chuck rotates about a rotation axis which is parallel to the sliding direction.
A tool-bearing turret is mounted on-board the second slide, at an end of which a bead-breaking tool and a tyre-demounting tool are mounted, which are positioned on diametrically opposite sides with respect to an axis of symmetry which is perpendicular to the sliding direction.
The tool-bearing turret is rotatable about the axis of symmetry, such as to be blocked in a first operating position, in which it turns the bead-breaking tool to face the wheel-bearing chuck, or in a second operating position, in which it turns the tyre-demounting tool to face the wheel-bearing chuck.
In order to perform the bead-breaking operation, the wheel is blocked in a deflated condition on the wheel-bearing chuck, normally with the aid of special raising devices, while the tool-bearing turret is in the first operating position.
Following this, the chuck is set in rotation and the slides are reciprocally neared such as to press the bead-breaking tool against the flank of the tyre of the wheel, pushing the tool towards the inside of the rim channel, up to detaching the bead of the tyre from the edge of the rim.
The bead-breaking tool is generally a sturdy specially-shaped steel disc, which is destined to rotate idle about the axis thereof, such as not to damage the tyre which rotates solidly with the wheel on the chuck.
The bead-breaking disc is also inclined with respect to the rotation axis of the wheel-bearing chuck, such that during the sliding of the slides, the bead-breaking disc can substantially wedge itself between the flank of the tyre and the edge of the rim, facilitating the detachment of the bead.
As heavy-duty tyre-changing machines must be able to operate with tyres of very different shapes and sizes, the bead-breaking disc is generally connected to the tool-bearing turret by hinging means, which enable an inclination thereof to be made with respect to the rotation axis of the tool-bearing chuck, on the basis of the type of wheel to be operated on.
This regulation is done manually, and the bead-breaking disc can be blocked in a discrete series of different positions, to which correspond various inclinations, by manual blocking means, such as for example a bolt.
The regulation of the inclination is thus done before the true and proper stage of bead-breaking, during which the bead-breaking disc constantly maintains the previously-set inclination.
However, though the inclination of the bead-breaking disc is correctly set, the pressure exerted by the disc against the flank of the tyre does not always enable an effective detachment of the bead, especially when the bead is glued to the edge of the rim.
This difficulty is due both to the practical impossibility of setting a correct inclination of the bead-breaking disc for all wheels to be operated on, and to the fact that the pressure exerted is purely axially, and thus acts exactly in the direction of maximum resistance of the coupling between the tyre bead and the edge of the rim.
In the sector of light-duty tyre-changing machines, i.e. machines predisposed to deal with car wheels, there exist machines whose functional architecture is very similar to that of the heavy-duty machines described herein above.
These light-duty tyre-changing machines comprise a wheel-bearing chuck which rotates about a substantially-vertical rotation axis, and a vertical upright on which a tool-bearing arm is slidably mounted, at a free end of which a bead-breaking disc is hinged, which is destined to be pressed against the flank of the tyre of the wheel, following an axial movement of the tool-bearing arm towards the wheel-bearing chuck.
Further, one of these bead-breaking machines enables the inclination of the bead-breaking disc to be varied with respect to the axis of the wheel-bearing chuck during the axial displacement of the tool-bearing arm, such that the bead-breaking disc performs a roto-translating movement in contact with the tyre, facilitating the detachment of the bead from the edge of the rim.
This effect is obtained by means of a cam-follower organ which is connected to the bead-breaking disc and which follows the profile of a cam fixed on the machine upright such as contemporaneously to obtain a rotation of the bead-breaking disc about the hinge axis thereof with the tool-bearing arm.
Though it improves the bead-breaking stage, this solution does not resolve the drawbacks which have been mentioned in relation to heavy-duty tyre-changing machines.
The bead-breaking disc is constrained to perform a predetermined and a constant roto-translating movement which is effective with a certain type of wheel but which cannot be modified to operate with wheels having different shapes and/or dimensions, if not by physically replacing the cam or other structural components of the tyre-changing machine.